1. Field of the Invention
This invention relates generally to the field of electric well logging. More particularly, the invention relates to methods for determining sonde errors in electromagnetic induction and propagation tools.
2. Background Art
Electromagnetic (EM) induction and propagation tools have been used for many years to measure the resistivity of earth formations surrounding a borehole. EM logging tools measure the resistivity (or its inverse, conductivity) of the formation by inducing eddy currents in the form ations in response to an AC transmitter signal. The eddy currents induce secondary magnetic fields that in turn induce voltages in receiver antennas. Because the magnitudes of the eddy currents depend on formation conductivities, the magnitudes of the received signals reflect the formation conductivities.
Ideally, an induction or propagation logging tool should read zero in a zero-conductivity medium. However, EM logging tools include conductive materials (such as sonde wiring, quadraxes, bulkheads, electrodes for spherical focused logs mounted on the induction sonde, etc.) that can respond to the magnetic field generated by the transmitter. The induced eddy currents in these metal parts produce a constant signal in the receivers. Consequently, an induction or propagation tool will not read zero in a zero-conductivity medium. This self-signal is referred to as a sonde error. In principle, the sonde error can be determined by suspending the tool in free space far from any external conductive material. However, most induction or propagation tools are designed to measure far into the formation (i.e., deep depth of investigation) to determine formation resistivity beyond the zone of invasion. These “deep-reading” tools render free-space determinations of sonde errors impractical. For this reason, a method for sonde error correction has been disclosed in U.S. Pat. No. 4,800,496, which is issued to Barber et al. (“the Barber patent”) and assigned to the assignee of the present invention. This patent is incorporated by reference in its entirety.
The methods disclosed in the Barber patent provide sonde error corrections for conventional induction tools by making measurements at two different distances from the earth. An algorithm then relates the difference in the voltages at the two heights to the earth signal. Finally, the earth signal is subtracted from the sonde reading to obtain the true sonde error. The true sonde error can then be used to correct the logging measurements.
Conventional induction logging tools have their transmitters and receivers arranged in a manner such that their magnetic moments are aligned with the longitudinal axis of the tools. These longitudinal induction array tools induce eddy currents in loops that are perpendicular to the longitudinal axes of the tools. Conventional induction tool cannot provide accurate resistivity estimates of formations with anisotropy. Formation anisotropy results from the manner in which formation beds were deposited by nature. Formations containing hydrocarbons often exhibit anisotropy in formation resistivity. In such formations, the horizontal conductivity, σh (or resistivity, Rh) in a direction parallel to the bedding plane differs from the vertical conductivity, σv, (or resistivity, Rv) in a direction perpendicular to the bedding plane.
To measure conductivity of a formation with anisotropy, new EM induction or propagation tools typically include transverse arrays that have transmitter and/or receiver antennas arranged such that their magnetic moments are substantially perpendicular to the axis of the instrument. See e.g., Moran and Gianzero, “Effects of Formation Anisotropy on Resistivity Logging Measurements,” Geophysics, 44, 1266–1286 (1979). Transverse array tools include triaxial array tools. Each triaxial array in these tool includes three orthogonal transmitter coils and three receivers coils in the same orthogonal orientations. In operation, the triaxial transmitter is energized in three orthogonal directions. Individual receiver coils, aligned in the same three orthogonal directions, then measure the voltages induced by eddy currents flowing in the surrounding formations. Examples of tri-axial tools may be found in U.S. Pat. No. 3,510,757 issued to Huston, U.S. Pat. No. 5,781,436 issued to Forgang et al., U.S. Pat. No. 3,609,521, issued to Desbrandes, U.S. Pat. No. 4,360,777, issued to Segesman, and U.S. Pat. No. 6,553,314 issued to Kriegshäuser, et al.
In contrast to a conventional induction tool, which induces eddy currents flowing in planes perpendicular to the longitudinal axis of the tool, a transverse array induces eddy currents that flow in planes parallel to the longitudinal axis of the tool. A triaxial array has a transmitter and a receiver, each having three coils arranged in orthogonal directions. Therefore, there are nine couplings between the transmitter and the receiver in a triaxial array. Each coupling is sensitive to different directions of eddy current flows. Furthermore, each EM induction or propagation tool typically includes multiple arrays. Accordingly, sonde error calibration for EM tools having transverse or a triaxial arrays is more complicated than that for a conventional induction tool, and it is desirable to have methods that can calibrate sonde errors for an EM tool having transverse or triaxial arrays.